One reason to cause this problem is the user account :TFSSERVICE, either locked or password expired.
In the event log, you will got following error event:
Application pool Microsoft Team Foundation Server Application Pool has been disabled. Windows Process Activation Service (WAS) encountered a failure when it started a worker process to serve the application pool.
1. Open Team Foundation Server admin console, find the action, "reapply the account", and input password again.
2. Reset IIS server